1. Field of the Invention
The invention relates to control circuits, and in particular, to control circuits for electric motors which have facilities for terminating the operation of the motor in the event of a defective condition. For example, such an electric motor can be a three-phase compressor motor of a refrigerating system, an air conditioning system, or the like.
2. Description of the Prior Art
The prior art, as exemplified in U.S. Pat. Nos. 2,945,133, 3,102,677, 3,155,878, 3,290,576, 3,312,081, 3,329,869, 3,366,843, 3,377,816, 3,404,313, 3,416,060, 3,526,809, 3,555,356, 3,562,587, 3,577,741, 3,599,439, 3,648,074, 3,673,811, 3,693,047, 3,712,991, and 3,787,793, contains a number of control circuits including circuits responsive to overheating of a motor winding or to loss of oil pressure for terminating the energization of a motor.
In addition to the above-mentioned patents, a prior art motor protection circuit is illustrated in FIG. 1, wherein a triac 10 is connected in series with a winding 11 of a contactor, indicated generally at 12, and a control switch, such as a thermostat switch 13, between AC power terminals 14 and 15. The contactor 12 has normally open contacts 16, 17 and 18 connected in series with respective input lines 20, 21 and 22 from a three-phase power source 24 to a three-phase motor 26. A gate electrode of the traic 10 is connected by contacts 28 of a relay, indicated generally at 29, in series with a resistance 30 to the terminal 15. A transformer, indicated generally at 32, has a primary winding 33 connected across the terminals 14 and 15. A first secondary winding 34 of the transformer 32 is connected in a loop circuit with a winding 36 of the relay 29 and the anode and cathode electrodes of a silicon controlled rectifier (SCR) 37. Another secondary winding 38 of the transformer 32 has end taps 39 and 40 connected in a second loop circuit including a resistance 42, a resistance 44, and three parallel branch circuits which include, respectively, a thermosensing resistance 45 in series with a resistance 46, a thermosensing resistance 47 in series with a resistance 48, and a thermosensing resistance 49 in series with a resistance 50. The thermosensing resistances 45, 47 and 49 are lengths of positive temperature coefficient wires which are imbedded in the respective three-phase windings of the motor 26. A center tap 53 of the winding 38 is connected to the junction of the winding 34 with the cathode of the SCR 37. The gate electrode of the SCR 37 is connected to the anodes of diodes 54, 55 and 56 which have their cathodes connected, respectively, to the junction 58 of the thermosensing resistance 45 and resistance 46, to the junction 59 of thermosensing resistance 47 and resistance 48, and to the junction 60 of thermosensing resistance 49 and resistance 50. A resistance 62 and a capacitance 64 are connected between the end tap 39 and the gate electrode of the SCR 37. A capacitance 66 and a resistance 68 are connected in parallel across the cathode and gate electrodes of the SCR 37. Normally open contacts 70 of the relay 29 are connected across the resistance 42. A capacitance 74 is connected across the relay winding 36.
The circuit of FIG. 1 forms a bridge circuit with taps 39 and 40 of the secondary winding 38 forming the power nodes, the center tap 53 forming a first sensing node, and the junctions 58, 59 and 60 forming multiple second sensing nodes. The respective portions of the winding 38 on opposite sides of the center tap 53 form first and second arms of the bridge circuit; the resistors 42 and 44 and the thermosensing resistances 45, 47 and 49 form a third arm which divides into multiple branches in the bridge circuit; and the resistances 46, 48 and 50 form multiple fourth arms of the bridge circuit.
In operation of the prior art circuit of FIG. 1 under normal conditions, the triac 10 is conductive to allow the control switch 13 to control the operation of the contactor 12 and thus the motor 26. The transformer 32 is energized by current from terminals 14 and 15 to produce AC current from the secondary winding 38 through the resistance 62 and capacitance 64 generating a voltage across the resistance 68, the capacitance 66 and the gate-cathode electrodes of the SCR 37. The capacitor 64 phase shifts the current therethrough to lead the voltage across taps 39 and 53 and to compensate for the phase shift lag caused by the capacitor 66 which protects the SCR 37 against spurious high voltage pulses. The resistors 62 and 68 aid in dividing the voltage across taps 39 and 53 to present a suitable voltage to the gate-cathode electrodes of SCR 37. During positive half cycles, the SCR 37 is triggered conductive by the gate-cathode voltage to energize the relay winding 36 which closes the contacts 28 to render the triac 10 conductive. The capacitor 74 charges during positive half cycles to help maintain energization of relay winding 36 during negative half cycles when the SCR 37 is non-conductive.
Operation of relay 29 also closes contacts 70 to shunt the resistance 42 and set the characteristics of the bridge circuit responsive to temperatures equal to or above a first temperature of about 121.degree.C (250.degree.F). Under normal conditions, the temperature of the windings of the motor 26 will be below 121.degree.C. The voltages on the nodes 58, 59 and 60 due to the low temperature resistance values of thermosensing resistances 45, 47 and 49 are greater than the voltage on tap 53 by more than the required triggering voltage of the SCR 37. During the positive half cycles across SCR 37, the diodes 54, 55 and 56 remain nonconductive at voltages on the gate electrode of SCR 37 less than the respective voltages on the nodes 58, 59 and 60.
When the temperature of one or more of the thermosensing resistances 45, 47 and 49 rises above 121.degree.C, the voltage on one or more of the respective nodes 58, 59 and 60 is lower than the voltage on the gate electrode of SCR 37 required to trigger the SCR 37. The respective diode or diodes 54, 55 or 56 become conductive and shunt the gate electrode of the SCR during positive half cycles to prevent triggering and conductivity of the SCR 37. Thus, the relay 36 is deenergized and the contacts 28 are opened rendering the triac 10 nonconductive to deenergize the winding 11 and open the contacts 16, 17 and 18 preventing operation of the motor 26 to avoid heat damage to the windings aand insulation of the motor 26. When relay 29 is deenergized, the contacts 70 open to insert the resistance 42 into the bridge circuit which is thus reset to prevent reenergization of relay 29 until the temperature of all the thermosensing resistances 45, 47 and 49 fall below a second temperature of about 71.degree.C (160.degree.F); the increase in resistance in series with the thermosensing resistances 45, 47 and 49 lowers the voltage on nodes 58, 59 and 60 to maintain conductivity of the diodes 54, 55 and 56 to prevent triggering voltages on SCR 37. The time required for the motor windings to cool from 121.degree.C to 71.degree.C provides a time delay which can allow the cause of the overheating to correct itself; refrigeration compressor motors can be prevented from starting by a high refrigerant pressure in the compressor, and the delay can provide time for the pressure to bleed off.
Prior art motor protection circuits which sense the temperature of windings of a motor have been subject to long term calibration drift due to changes in ambient temperature of bridge resistors, transformer windings, bias resistors, phase shift capacitors, logic diodes, and the gate characteristics of SCR's; such calibration drift making the circuits unreliable in preventing motor winding damage or unnecessarily terminating operation of the motors unless special and expensive low drift parts are used. The prior art bridge circuits for multiple winding temperature sensing required a special multiple tap transformer which was expensive. Prior art circuits with multiple temperature sensing branch circuits did not provide complete isolation between the temperature sensing branch circuits. Also, the prior art circuits utilized a relay to provide a temperature differential, such relay being expensive and subject to chatter, vibration sensitivity, and variation of contact resistance thus affecting the basic calibration of the circuits. Further, the prior art thermosensing circuits required trimming of resistances and/or capacitors, or the employment of variable resistors which are often subject to failure.
In motor protection circuits where an electronic timing circuit is used to terminate operation of a motor after a duration of the absence of lubricating oil pressure; power failures, spurious voltage pulses, and intermittent operation of an oil pressure sensing switch can cause erratic and erroneous operation of motor energization control circuits.